1. Field of the Invention
The present invention relates to a method for producing a semiconductor device such as a semiconductor acceleration sensor.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 59-13377 discloses an electrochemical etching method for processing a diaphragm of a diaphragm silicon pressure sensor. This method employs an HF-based etching liquid. The etching liquid etches a silicon wafer when a voltage is applied with no flow of current. The etching is stopped if a flow of current is detected.
A KOH-based etching liquid is also employable. When a voltage is applied with no flow of current, the etching liquid etches a silicon wafer due to chemical reactions between KOH and silicon. Since the KOH-based etching liquid causes anodic oxidation of silicon to continuously produce a weak flow of current, it is difficult to clearly detect the end of the etching.
The electrochemical etching is useful to precisely form a thin part such as a diaphragm of semiconductor pressure sensors and acceleration sensors. Conventional electrochemical etching methods usually employ scribe lines as current supply paths to etch a wafer. Japanese Unexamined Patent Publication No. 61-30039 employs an n type high concentration diffusion layer to supply an etching current. To improve the efficiency of etching, a metal thin film having lower resistance may be employed instead of the diffusion layer. The metal thin film formed on scribe lines on a wafer, however, produces metal dust when a dicing blade dices the wafer into chips along the scribe lines. The metal dust will adhere to the chips to damage the properties thereof.
The Japanese Unexamined Patent Publication No. 61-30039 employs a silicon substrate made of upper and lower conductive layers having different conduction types. A high concentration diffusion layer serving as an electrode is formed on the substrate. The lower layer is electrochemically etched so that the upper layer is left to form a diaphragm.
This technique is effective to provide each chip with a diaphragm of uniform thickness when the chip is small. When the chip is large, however, the technique hardly achieves uniform thickness in the diaphragm. It is particularly difficult to form a diaphragm of uniform thickness when a thin epitaxial layer is employed to fabricate a thin diaphragm. This is because such a thin epitaxial layer has a higher horizontal resistance, and therefore, a central part of each chip area distal from scribe lines receives insufficient potential from a voltage supply source. Because of this, etching will not easily stop at the central part because an anodic oxidation film is hardly formed there.
The conventional methods have the problems of incorrectly detecting the end of etching, of erroneously stopping the etching and of scattering a metal film formed on scribe lines when a wafer is cut into chips along the scribe lines. Such scattered metal film will make semiconductor circuits on each chip defective, thus deterrorating the yield. The conventional methods are unable to form a diaphragm of uniform thickness from a thin epitaxial layer due to a high horizontal resistance of the layer.